Detroit – A new report on recalls of potentially deadly Takata air bag inflators shows that automakers have replaced only 43 percent of the faulty parts even though recalls have been under way for more than 15 years.

The report, issued Friday by an independent monitor who is keeping tabs on the recalls, also shows that auto companies are only about halfway toward a Dec. 31 goal of 100 percent replacement of older and more dangerous inflators.

The slow completion rate comes even though the National Highway Traffic Safety Administration began coordinating the recalls and phasing them in two years ago. Before that, the automakers were obtaining parts and distributing them on their own. Normally automakers fix 75 percent of vehicles within 18 months after the recall is announced.

The report brought criticism from a U.S. senator in Florida, whose state has seen three deaths caused by the problem and where automakers have fixed 41.7 percent of the 3 million affected inflators.

Takata uses the chemical ammonium nitrate to create a small explosion and fill air bags quickly in a crash. But the chemical can deteriorate when exposed to high humidity and temperatures and burn too fast, blowing apart a metal canister designed to contain the explosion. That can hurl hot shrapnel into unsuspecting drivers and passengers. At least 19 people have been killed worldwide and more than 180 injured.

The problem touched off the largest series of automotive recalls in U.S. history, with 19 car and truck makers having to recall up to 69 million inflators in 42 million vehicles. It also brought a criminal conviction and fine against Takata and forced the Japanese company into bankruptcy protection.

The report by independent monitor John Buretta says that as of Sept. 15, automakers have recalled 43.1 million inflators. Of those, only 18.5 million have been replaced, even though Takata recalls date to 2001.

In his report, Buretta concludes that there is “much room for improvement” in the Takata recalls. But he says that manufacturers are starting to make meaningful progress toward “developing sound strategic approaches.”

The automakers, he writes, are using different communications methods to reach owners such as door-to-door canvassing. They also are offering mobile repair and trying to use third parties such as independent repair facilities to speed up the process.

Sen. Bill Nelson, D-Florida, blamed the slow pace on a lack of leadership at NHTSA, which has been without its top administrator since the end of the Obama administration in January. “We still don’t have any leadership at NHTSA to ensure this stuff actually gets done by the automakers,” he said in a statement. “Until the agency gets a permanent administrator this recall is going to continue to drag on while the injury and death toll mounts.”

NHTSA said in a statement that the Takata recalls are unprecedented in size and complexity and have resulted in groundbreaking lessons that will help automakers reach their repair goals. The agency said it is monitoring the automakers’ progress and working to expand best practices to boost completion rates. The agency also has the authority to fine automakers that don’t make recall repairs in a timely manner.

“NHTSA will rely on its broad array of enforcement authorities and will take further action as appropriate,” the statement said.

Completion rates vary wildly by automaker, according to a separate document posted by NHTSA. Tesla was best at 78.6 percent, followed by Honda at 64.8 percent. Mercedes-Benz was the worst at 2.3 percent, followed by Karma at 9.9 percent.

Automakers initially were slowed by a lack of replacement inflators as Takata and other manufacturers ramped up manufacturing. But for many such as Honda, Takata’s largest customer, ample parts are now available.

NHTSA coordinated the parts distribution, sending them first to southern states with higher temperatures and humidity. The recalls will be phased in over roughly the next three years.